• To create a snapshot of BACKING_IMG.qed run the following command. Then use SNAP_IMG.qed as the source image in the vm. All changes will be written to SNAP_IMG.qed and BACKING_IMG.qed will be unmodified.
• Running the same command will create a new snapshot and discard all changes to BACKING_IMG.qed
• qcow2 and qed support snapshots

qemu-img create -f qed -b BACKING_IMG.qed SNAP_IMG.qed

• You can chain snapshots so that following command would make a snapshot of a snapshot

qemu-img create -f qed -b SNAP_IMG.qed SNAP_OF_SNAP_IMG.qed